1. Field of the Invention
The present invention relates to an acoustic absorber that is particularly suitable for engines of aircraft and, in particular, has a honeycomb-shaped 3-dimensional structure which absorbs engine noise when used in an engine. The invention further relates to a particular use of folded honeycomb structures.
2. Discussion of Background Information
Engines in general, and in particular aircraft engines, have the problem of a high noise emission. There is a strong noise emission in the engine intake, particularly with engines having a high partial-flow ratio. Blade tip speeds of more than Mach1 cause, e.g., shock waves that excite acoustic eigenmodes in the intake duct. These modes radiate outward very effectively and cause a very loud noise. This noise, also called “buzz-saw noise,” is perceived particularly intensely during the take-off phase of an aircraft in the vicinity of the airport. But the noise also penetrates into the passenger cabin of the aircraft itself, and is perceived by the passengers, in particular in the front section. Further noise proportions occur with frequent passage of a blower or fan blade, as well as the harmonics from the blower or fan blade. Additional noise proportions also come from broadband noise as well as from other engine stages such as, e.g., a compressor.
In order to control this noise in the intake duct or in an engine's partial-flow duct, absorbing surfaces are usually attached in the area of the engine intake as passive sound absorbers. So-called “single degree of freedom” or SDOF absorbers are thereby predominantly used, and these absorbers are essentially composed of a honeycomb structure covered by a perforated sheet or a fine-meshed wire netting. The honeycomb structure acts as a resonator, whereby the structural depth of the honeycomb structure is adapted to the wavelength and is a quarter of the wavelength. Thus, SDOF absorbers of this type are particularly effective at a resonance frequency.
Absorbers were developed that are composed of two or three SDOF absorbers connected in series to render possible a wider broadband noise damping. Absorbers of this type are also referred to as 2-DOF absorbers or 3-DOF absorbers. Furthermore, attempts were made to connect absorbers of various structural depth in parallel.
For the purpose of illustration, FIG. 2a shows a known SDOF absorber with a plurality of honeycombs 1 arranged between a lower surface layer 2 and an upper surface layer 3. FIG. 2b shows a known 2-DOF absorber with two layers of honeycombs 1a, 1b lying on top of one another under the surface layer 3.
With the known absorbers, there is the problem that large-area curved structures can be manufactured only in a very complex manner, which entails high costs. Current honeycomb-core absorbers in engines have the particular disadvantage that they do not cover the entire surface of the intake. One reason for this is the so-called saddle effect, which is the reason that honeycombs cannot be curved arbitrarily. Thus the effective surface is not utilized, and the noise in the engine intake cannot be absorbed or reduced sufficiently.
For the purpose of illustration, FIGS. 3a and 3b show the saddle effect. When a flat honeycomb structure according to FIG. 3a is attempted to be curved, then the geometry of the honeycombs forms a saddle-like structure with a first or inner radius of curvature ρ1 and a second radius of curvature ρ2, which forms a saddle-like indentation of the honeycomb structure.
There is the additional problem that the honeycombs cannot be interlocked with one another, which makes a seamless manufacture even more difficult. Furthermore, because of their manufacturing process, the honeycombs cannot be positioned precisely, which is why, e.g., required perforation holes are covered in many cases. This applies in particular to MDOF absorbers (MDOF=multiple degree of freedom) with several degrees of freedom, which absorbers require a very precise positioning of the intermediate layers. This means that the honeycomb layers lying on top of one another must be positioned particularly precisely. With absorbers of this type, the above-mentioned saddle effect is of particular impact because it causes a lateral displacement of the respective honeycomb to the surface layers and intermediate layers.
So far MDOF absorbers have always had to be manufactured by sandwich constructions with several cores. This increases costs and requires a very precise alignment of the honeycombs to the perforated intermediate layers. Furthermore, honeycombs are set to a fixed cell size. This means that it is not possible to realize absorber structures with variable volumes.